Conversion of geometrical isomers of menthol to racemic menthol by silica catalysts



2,327,498 Patented Mar. 18, 1958 CQNVERSIGN 9F GEOP/IETRICAL ISOMERS OF MENTHGL TO RACEMIC MENTHOL BY SEICA CATALYSTS Robert R. Bottoms, Crestwood, Ky., assignor to National Cylinder Gas Company, Chicago, 111., a corporation of Delaware N Drawing. Application May 27, 1957 Serial No. 661,599

9 Claims. (Cl. 260-631) JHOH CH JH(CHB)2 but also geometrical isomers of menthol, namely, neomenthol, isomenthol, and isoneomenthol. These geometrical isomers of menthol have the same empirical and graphic formulas as menthol and exhibit essentially the same chemical properties, although they differ in their physical properties. Racemic menthol is the only product of any commercial value since it can be converted to natural l-menthol by resolution procedures. The geometrical isomers of racemic menthol have no commercial value and it is therefore highly desirable that these lay-products of the synthesis of menthol be converted into useful materials.

It is the object of this invention to provide a method for converting geometrical isomers of menthol to racemic menthol. It is another object of this invention to provide a method for converting the undesirable geometrical isomers of menthol which are produced in menthol syntheses into useful materials. It is a further object to increase the yields of menthol in commercial syntheses rom thymol, menthone, pulegone and piperitone. Other objects of this invention will be apparent from the following disclosure.

It has been known that geometrical isomers of men thol may be partially converted to racemic menthol by prolonged heating with alkaline reagents such as alkali metal menthoxides or alkaline-earth metal menthoxides. By the present invention it has been discovered that geometrical isomers of menthol may be converted in high yield to racemic menthol by heating with a catalytic quantity of finely divided silica, SiO or materials which are convertible to silica upon heating. The racemic menthol so formed may then be isolated by fractional distillation or by fractional crystallization or by combinations thereof in the conventional manner employed in the prior art. The geometrical isomers of menthol which are not converted to racemic menthol may then be retreated in the same manner for the production of further quantities of racemic menthol. in this way it is possible to convert geometrical isomers of menthol to racemic menthol with very little loss of material. This highly efiicient method contrasts with the prior art methods used heretofore for the conversion of the geometrical isomers of menthol to racemic menthol. By such procedures, the sodium, potassium or other alkali or alkaline-earth metal derivatives of the geometrical isomers of menthol are prepared and then these materials on prolonged heating for periods of 24 hours or more at high temperatures in the range of 200 to 210 C. are partially isomerized to dl-menthol. After the heating period it is necessary to treat the reaction mixture with water or dilute acid to remove the alkali or alkalineearth metals, and thereafter the reaction mixture is fractionated to separate the desired dl-menthol. In the prior art process, about 28% of the isomers were converted to menthol at each treatment.

The process of the present invention has the important advantage that the catalyst for the isomerization is readily separated from the reaction product by simple sedimentation and can be reused for subsequent runs. Other important advantages are: (l) a larger proportion of the isomeric menthols is converted to racemic menthol in each treatment; (2) a shorter period of heating may be used; and (3) the catalyst for the conversion is an inexpensive, readily available material. All of these factors result in a vastly more economical method of producing racemic menthol from mixtures of geometrical isomers of menthol.

Broadly, the invention is carried out by heating a geometrical isomer of menthol or a mixture of geometrical isomers of menthol with finely divided silica for a period of time ranging from one to three hours to as long as twelve to fifteen hours, at a temperature above C. and preferably in the range of 180 to 200 C. Temperatures above 210 Q are to be avoided because of decomposition of the menthol. The amount of silica used in the methods can be relatively small since the ilica is a catalyst and is not consumed, destroyed or inactivated during reaction. Generally, 1% to 3% of silica based upon the weight of geometrical isomers of menthol used has been found to be satisfactory, although obviously larger amounts may be successfully employed. The silica may be any of the common forms of SiO; such as micro crystalline silica mined in Arkansas and sold under the trademark Novocite. Finely ground silica sand, diatomaceous earth, kieseiguhr, pulverized silica gel, fullers earth and like silica-containing materials are suitable catalysts. Hydrates of silica such as the silicic acids and silica hydrogels are also operative. Of the above, preferred catalysts include ground silica sand or micro crystalline mined silica, although the other ma terials are equally effective.

in the actual operation of the process a charge of liquid geometrical isomers of menthol is agitated and the catalytic amount of silica is dispersed therein. It is desirable that the catalyst be in intimate contact with the menthol isomers, and this is best achieved by mechanical agitation. The mixture is heated to about C., continuous agitation being maintained. After the mixture has been agitated and heated for the appropriate period of time, the reaction mixture is allowed to cool and the sediment of silica is removed by decantation. The mixture of menthol and menthol isomers is then subjected to fractional distillation. The menthol distilling at a higher temperature than its geometrical isomers. If desired, the racemic menthol can be obtained in a high state of purity by crystallization procedures.

The following example illustrates one embodiment of the invention, but it is to be understood that the invention is not to be limited thereto.

It will be apparent to those skilled in the art that many modifications in the conditions and amounts; of materials may be made without departing from the scope of-the invention. Relative amounts of materialsare fgiven'in parts byweight and temperatures, are given iridegrees centigrade. 7

Example Two thousand parts of'a mixture of geometrical isomers ofmenthol is placed in a still and agitated with a revolving agitator. Twenty parts of finely pulverized silica sand is added and the mixture is heated with continuous agitation at 190 C. for a period of eight hours. The agitationand heating arethen discontinued and the catalyst allowed to settle while the reaction mixture is cooling. The clearorganic liquid'is decanted from/thecatalyst and transferred to' another still an d distilled-un der reduced pressure through an efiicient fractionating column. The first cut obtainedjin the temperature range of 100 to 105 Cyat l"mi llimeters pressure consists largely of the more liquid'menthol isomers. This cut generally comprises about 50% ofthe charge. A second cut distilling at 105 to 107 C. at 15 millimeters pressure is then collected This .fraction comprises about 50%; of the charge and consists largely of "racemic menthol; The latter may thenvbe chilled to induce crystallization and'the crystallized material is centrifuged to obtain high purity crystalline menthol. The yieldof such crystalline material is about 40% of'the amount 'of geometrical isomers of menthol used.

The first cut of geometrical menth lisomersis then returned to the first still containing the finely pulverized'silica sand and further agitated and heated at- 190 C. By continuing the isomerizationin this fashion it is possible to convert at least 8 5% of the geometrical isomers of menthol 'to crystalline racemic menthol.

What is claimed'as new and desired .to be securedby Letters Patent of the United. States is:

l. A method of converting a geometrical isomer of menthol to menthol which comprises heating said geometrical'isomer ofjmenthol with a silica-containing material' at a temperature not greater than about 210 C.

2. A m ethod of converting a geometrical isomer of menthol to mentholwhich comprises heating said ge- 4: ometrical isomer of menthol with silica at a temperature not greater than about 210 C.

3. A method of converting a geometrical isomer of menthol to menthol which comprises heating said geometrical isomer of menthol with a silica-containing material at a temperature in the range of about 150 C. to; about 210 C. V I 4. A. method. of. convertinga. geometrical isomer of menthol to menthol which comprises heating said geometrical isomer. of menthol with-finely divided. silica at. a temperature above, about 150 C. but'not greater than about 210 C. and separating the racemic'menthol thus formed.

5. A method of convertingageometrical isomer of menthol to mentholwhich comprises heating said ge-v ometrical isomer ofmentholjwith finely divided silica at a temperature in the range of about 180 C. to about 200 C. and separating the racemic menthol thus formed.

6. A method of. cojnvertinga geometrical isomer of menthol to menthol. which, comprises. heating said ge ometrical isomer of menthol with finely divided. silica atatemperature in therangeof; about 180 C. to about 200 C. and separating the racemic menthol thus formed.

by fractional distillation.

7. A. method 0t converting a geometrical, isomer of menthol to menthol whichcomprises. heating said: geornetrical isomer of menthol with. finely, divided: silica ata temperature in ;the.range ofl80 C. to 200 C. and

8. A method of converting racemic neomenthol to V racemic menthol which comprises heating neomenthol with finely ground silica; sand at a temperature in the range of 180 C. to 200? C. and separating the racemic menthol thus formed by fractional distillation and by fractional crystallization. a

9. The method of converting racemic isomenthol to racemic menthol which comprises heating isomenthol with finely ground'silica sand'at a temperature in the rangev of 180 C; to 200 C. and separating the racemic menthol thus. formed by fractional distillation and by fractional crystallization.

No references cited. 

1. A METHOD OF CONVERTING A GEOMETRICAL ISOMER OF MENTHOL TO METHOL WHICH COMPRISES HEATING SAID GEOMETRICAL ISOMER OF MENTHOL WITH A SILICA-CONTAINING MATERIAL AT A TEMPERATURE NOT GREATER THAN ABOUT 210*C. 